i think they had a script for the next season, but i don't think many people liked it so they just stopped. they then turned that scrpt later on into Justice League: Crisis on Two Earths. i loved the series too, and they left some questions unanswered.

Starcrossed - the 2nd season finale was originally intended to be a series finale for whatever reason, but given how it well did they decided to extend the series. So JLU came about and with Cadmus Arc there was interest to continue beyond that as well.

Of course Epilogue was fully made with the intention that it be the series finale JLU. This was not the case so then they made another season of JLU which ended with Destroyer. In effect, between Justice League and JLU, there were 5 seasons total.

Justice League: Crisis on Two Earths spun off from the idea of making Justice League: Worlds Collide. Many of the same people who worked behind the scenes on Justice League were still involved with multiple DCU animated productions including Crisis on Two Earths.

Another DCU Justice League film worth watching over and over is Justice League: The New Frontier based on the graphic novel volumes of the same title.